1. Technical Field
The present disclosure relates to a microelectromechanical sensor with out-of-plane sensing and to a process for manufacturing a microelectromechanical sensor.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has increasingly continued to spread in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one mass connected to a fixed substrate by springs and movable with respect to the substrate according to pre-set degrees of freedom. The movable mass and the substrate are capacitively coupled through plurality of respective electrodes set facing one another so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied.
In a first family of microelectromechanical sensors (also referred to as “sensors with in-plane sensing”), the movable mass is constrained in such a way as to translate or rotate parallel to the substrate. More precisely, in devices of this type the movable mass can translate along one or two axes parallel to the substrate, or else rotate about an axis perpendicular thereto. The electrodes are generally obtained by definition of conductive layers formed on the substrate and possibly total or partial removal of sacrificial layers.
A second family of microelectromechanical sensors (also referred to as “sensors with out-of-plane sensing”) comprises devices in which the movable mass is constrained so that its distance from the substrate can vary in response to stresses according to one axis. In particular, the movable mass can translate along an axis perpendicular to the substrate or else rotate about an axis parallel to the substrate. The electrodes generally face one another, through the space that separates the movable mass from the substrate, so as to alternatively approach or recede according to the movements of the movable mass. To form the electrodes, conductive layers are laid and shaped, which are separated from a sacrificial dielectric layer. The movable mass is formed on the conductive layer formed last. Removal of the sacrificial layer allows for release of the movable mass, creating a gap between the movable mass itself and the substrate.
There then exist microelectromechanical sensors in which the movable mass has more than one degree of freedom. In this case, sensing can be of the in-plane type along a first axis and of the out-of-plane type along a second axis.
Sensors with sensing of the out-of-plane type present limits due to the fact that the dimensions of the gap and the distance between the electrodes are basically imposed by the thickness of the sacrificial layer. On account of the structure of known sensors of the out-of-plane type, there is hence little freedom in the choice of the geometry of the electrodes and of the dimension of the gap. Since the performance (in particular, sensitivity, accuracy, and full-scale values) are affected to a determining extent by these parameters, also the variety of microelectromechanical sensors with sensing of the out-of-plane type is limited.